An optical waveguide device using an electrooptical crystal such as a LiNbO3 (LN) substrate, a LiTaO3 substrate or the like is being developed. These optical waveguide devices may be formed through processes of providing an electrode near an optical waveguide after forming the optical waveguide by forming a metal film such as Ti on a part of a crystal substrate and thermally diffusing the metal film or by proton-exchanging in benzoic acid after patterning. Japanese Patent Application Publications No. 2009-157114, No. 2006-301612 and No. 2006-195256 disclose a Mach-Zehnder optical modulator as the optical waveguide device.